Chapter 12: Charging Methods and Techniques: General Requirements and Selection of Chargers pdf

12
Charging Methods and Techniques:
General Requirements and Selection
of Chargers
E. WEHRLE
12.1 THE BATTERY’S REQUIREME NTS FOR THE CHA RGER
Charging of batteries must be conducted with direct current. Alternative current or
rotary current have to be transformed. Mostly semiconductor rectifiers are employed
for this task. Methods for battery charging vary with demand and the charging time
is of great importance. The charging devices can be divided into those that ch arge
above gassing voltage and those that do not. Chargers that exceed gassing voltage
during charging are employed for charging one battery at a time, while the ones that
do not exceed gassing voltage can be used for parallel charging of several batteries.
The chargers that exceed gassing voltage attain short times for recharge, whereas
with chargers that do not charge above gassing voltage very long charging times
must be expected.
12.2 TECHNICAL DATA AND TERMS
Technical data on the charging process for lead-acid and NiCd accumulators are
summed up in Table 12.1. The following illustrates the most common technical terms
applied in connection with charging techniques (1).
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Table 12.1 Technical data for charging of lead-acid accumulators.
Line
Traction batteries Stationary batteries
Automotive
GiS PzS Gro GroE OPzS batteries
1 Rated capacity (C
n
)C
5
C

5
C
10
C
10
C
10
C
20
2 Charging coefficient 1.17 1.2 1.1 1.1 1.2 1.15
3 Energy efficiency after C
n
has been
drawn, standard values
0.70 0.68 0.75 0.75 0.68 0.75
4 Maximum permitted charging currents
per 100 Ah nominal capacity (A)
(a) current constant upon reaching
gassing voltage (I characteristic)
5 8.5 5 5 10
(b) decreasing current (W characteristic)
allowed at 2.4 V/cell 8 12 12 7 12
at 2.65 V/cell 4 6 6 3.5 6
(c) nominal current of the charger for (b)
at 2.0 V/cell (DIN 41774)
16 24 18 14 24
5 Maximum for the final charging phase
allowable for 2.5 days max., e.g. for
IU characteristic (A)
23322

6 Float charge current (see line 10) (mA) 40–100
7 Maximum initial current at 2.4 V/cell
and 208C (688F) (U characteristic),
tolerance + 10% (A)
100 80 80 80 80 160
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Charging voltages (V/cell)
8 Initial voltage at W characteristic and
current as in 4(c)
Dependent on type and size between 2.1–2.15
9 Charge end voltage at currents as in 4(c)
and (b)
Dependent on type and size normally 2.6 to 2.7 V and for old and warm batteries 0.2 V/cell less
10 Float charge current (see line 6) 2.20–2.25,
11 Trickle charge voltage 2.25–2.35
12 Constant voltage for IU charging 2.40 2.35 2.40 2.35 2.35 2.40
2.40 2.40
13 Secondary charging period (h)
at Wa characteristic 4.0 4.5
at WOWa characteristic dependent on
the initial current
4.5–5 5–5.5
at IOIa characteristic dependent on the
initial current
4.5–5 5–6
at IUIa characteristic and end of charge
current as in 4(a)
3.5 4.0
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12.2.1 Battery Capacity, Discharge Current, and Charge Current

Electrical batteries are DC storage systems that can either store or produce electrical
energy by chemical transformations. The process of storing energy is called
‘charging’, whereas the production of energy is called ‘discharge’. The chemical
transformations are proportional to the amount of current consumed, respectively
produced, in Ah, corresponding to Faraday’ s laws. Therefore the size of a battery is
given in Ah (amperes (A) 6 time (h)).
As the capacity is depen dent on the discharge current and the duration of
discharge, it is not a constant value. This can be derived by the designation given by
the manufa cturers. The nominal capacity is given for 5 hours discharge time (C
5
) for
vehicle batteries and NiCd batteries; whereas for stationary batteries (also common
for gas-tight NiCd batteries) the 10-hour discharge capacity (C
10
) is given; and for
starter batteries, motorcycle batteries, and small lead-acid accumulators the capacity
for a 20-hour discharge (C
20
) is given. A C
5
of 100 Ah signifies that this battery
produces 100 Ah during 5 hours of discharge and the 5-hour discharge current is
I
5
¼ 100/5 ¼ 20 A.
The corresponding discharge current (I
5
,I
10
) is also a measure for the charging

current. If a charging current of 2 6 I
5
is mentioned, this means that charging is
conducted with twice the 5-hour discharge current. For a capacity of 100 Ah this
amounts to 2 6 100/5 ¼ 10 A.
12.2.2 Charge Coefficient
The ratio of amount of current needed for full recharge to the drawn current is called
the charge coefficient. It amounts to 1.1–1.2 for lead-acid batteries depending on
their design and between 1.2 and 1.4 for NiCd accumulators (see also Tables 12.2
and 12.3).
During every charging process a part of the applied amount of energy is lost,
especially above the gassing voltage, through the process of chemical decomposition
of water and hydrogen in the electrolyte. Therefore a greater amount of energy must
be applied for charging than has been drawn prior to recharge. For example, given a
battery with a nominal capacity of 125 Ah; 80% discharged (100 Ah); with a charging
coefficient of 1.2; in order to attain fully charged state, 100 Ah 6 1.2 ¼ 120 Ah have
to be provided.
12.2.3 Charging Time
The given charging times are idealized calculated values presuming that all battery-
and rectifier-specific data are constant. Practically such conditions are not met as, for
example, mains fluctuations influence uncontrolled chargers; aging of the battery
and variant temperatures also have influence.
Variance of the electrolytes’ tempe rature by 108C (188F) (reference tempera-
ture for traction batteries 308C (86 8F), for stationar y batteries 208C (688F) and for
starter batteries 278C (80.68F)) changes the charging time by 1 hour. If the
temperature is lower than the corresponding reference temperature as above, then
charging is prolonged, whereas higher temperature shorten charging time. As these
disturbing variables cannot be controlled, they are not considered for calculations of
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Table 12.2 Technical data for charging NiCd and NiFe accumulators.

Line
Nickel cadmium Ni/Fe
R T TS F RE TNE
1 Rated capacity (C
n
)C
5
C
5
C
5
C
5
C
5
C
5
2 Charging coefficient 1.4 1.4 1.4 1.2 1.4 1.4
3 Energy efficiency after C
n
has been drawn,
standard values
0.50–0.55 0.55–0.60 0.60 0.75 0.45–0.50 0.50–0.55
4 Maximum permitted charging currents
per 100 Ah nominal capacity above
gassing voltage (A)
a) constant current (I characteristic) About 20–30 A limited by heating up 10 About 20–30 A limited by heating up
b) current decreasing
a
current decrease current decrease

allowable at 1.5 V/cell 40–50% 8
allowable at 1.6 V/cell 6 30–40%
c) nominal current of the charger as in b)
at 1.2 V/cell
20
5 Lowest possible charging current (A) — — — — 7
6 Float charge current (see line 9) (mA) 20–60 100–300 — —
Charging voltages (V/cell)
7 Initial voltage dependent of type, size, and
current 1.3–1.4 1.3 1.4–1.6
8 Charge end voltage dependent on type,
size, and current 1.6–1.85 1.6 1.7–1.85
9 Float charge voltage (see line 6) 1.38–1.40 1.36 — —
10 Trickle charge voltage dependent on type 1.4–1.5 1.4 — —
11 Buffer voltage at deactivation (vehicles
11 and train lights) 1.6 1.5 — —
12 Constant voltages for IU charging 1.6–1.7 1.5 1.7–1.75
13 Secondary charging time (h)
for Wa characteristic, I
N
¼ I
5
5.5 1.5 1.5 2.5
for WoWa characteristic — — — 3.5
a
For R-, T-, and TS-type cells the W characteristic according to DIN 41775 with variable niveau. For F-type cells a W characteristic is employed, but adjusted by a ratio of
1.2:2.
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the charging time. A variance of + 0.5 hour of the charging time should therefore be
expected.

12.2.4 Gassing Voltage
The voltage above which a battery shows significant gassing action is termed ‘gassing
voltage’. In reality the following values are encountered:
. 2.40 V/cell for lead-acid batteries.
. 1.65 V/cell for NiCd batteries, series T.
. 1.60 V/cell for NiCd batteries, series TS.
. 1.70 V/cell for NiCd batteries, series R.
. 1.50 V/cell for NiCd batteries, series F.
NiFe batteries show signs of gas emission immediately upon charge activation, but
also in certain amounts during open circuit and discharge operation.
12.3 CHARACTERISTIC CURVES
The charging methods differ with respect to their current and voltage characteristics
during charging and with the corresponding charging time. DIN 41 772 is the
standardization for charging device characteristics.
A characteristic of a charging device is coordination of the DC voltage and the
current valid for the given type of load.
The following progressions of characteristics have been determined by DIN 41
772 and fitted with the corresponding initial:
. Decreasing (taper) characteristic: W.
. Increasing characteristic: S.
Table 12.3 Allowed values for the charging current upon reaching gassing voltage for
different types of cells.
Cell type (1)
Nominal
capacity
Current (A) per 100 Ah nominal capacity for
charging method
1 max. 2a max. 2b max. 3 max.
GiS, PzS C
5

5842
Gro (vehicle) K C
5
10 14 7 3
Gro/GroE (stationary) C
10
8,5 12 6 3
OPzS C
10
5 7 3,5 2
Starter battery C
20
10 12 6 2
Charging method 1: charging with constant current and deactivation upon reaching fully charged state (Ia
characteristic).
Charging method 2: charging with decreasing current and deactivation upon reaching fully charged state.
2a: allowed current at 2.4 V/cell.
2b: allowed end-of-charge current at 2.65 V/cell.
Charging method 3: allowed end-of-charge current without deactivation for up to 3 days charging time.
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. Limited characteristics: (I), (U).
. Constant characteristic: I, U.
. Assembled characteristics: e.g., IU, IUW, IO, la.
These abbreviations help describe the static behavior of the rectifier. Abbreviations
for add itional information are, e.g., 0, e, and a. Figures 12.1 and 12.2 show the most
important modifications of charging characteristics.
Charging characteristics are generally influenced by external disturbances, such
as variances of the mains voltage, its frequency, or the surrounding temperature.
Special devices can largely diminish these influences. This is applied for constant and
limited characteristics. The tolerances for constant charact eristics must, if not stated

otherwise, remain within the following marginal values:
. Mains voltage; + 10%
. Mains frequency; + 2%
. Ambient temperature; 0 to 408C (32 to 1048F)
. Internal device temperature; 0 to 458C (32 to 1138F)
The operating range for which the characteristics are valid can be found in the
instruction manuals.
Figure 12.1 General charging characteristics.
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12.3.1 Decreasing (Taper) Characteristics (W Type)
A characteristic is termed decreasing when the voltage decreases with increa sing
current (type W).
12.3.2 Increasing Characteristics (S Type)
A characteristic is termed increasing when the v oltage increases with increasing
current (type S).
12.3.3 Limited Characteristics
Characteristics which independent of external disturbances do not vary by more
than +10 from their nominal values are termed ‘limited characteristics’.
1. If the desired limited value is a voltage, then a limited voltage characteristic
is at hand.
2. When the desired limited value is a current, then a limited current
characteristic is at hand.
Figure 12.2 Modification of the I and W characteristics.
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12.3.4 Constant Characteristics
Characteristics which independent of external disturbances do not vary by more
than +2% from their nominal values are termed ‘constant characteristics’.
1. When the desired value is a voltage, then a constant voltage characteristic
is at hand (type U).
2. When the desired value is a current, then a constant current characteristic

is at hand (type I).
12.3.5 Assembled Characteristics
An assembled characteristic is at hand if different characteristics pass over into one
another continuously or by a step (types WOWa, IU, SU).
12.4 EMPLOYMENT OF CHARGING METHODS
12.4.1 Installation and Operation of Batteries and Chargers
DIN 57 510/VDE 0510 (3) deals with operation and installation of batteries and
chargers.
12.4.2 Demands of Vented Lead-Acid Accumulators
The most important feature of chargers for lead-acid accumulators is the current
being limited when the gassing voltage (2.4 V/cell) is reached. When reaching this
value, the charging current is partially employed for decomposition of the
electrolytes’ water, and heat is excessively produced. Therefore the charge current
when the gassing voltage is reached has to be reduced to the values permitted by the
battery manufacturer.
12.4.3 Demands of the Maintenance-Free Lead-Acid Battery
In order to prevent the formation of gas inside the battery, charging may not be
conducted above the gassing voltage. The charging voltage is limited to 2.35 V/cell
for cyclic operation.
12.4.4 Demands of Vented Nickel/Cadmium Batteries
Here the current must not be reduced above gassing voltage (exception: cells with
sintered electrodes), but the allowed temperatures of 458C (1138F) must be respected
(for cells with pocket electrodes 358C (958F).
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12.4.5 Charging Lead-Acid Batteries According to the
W Characteristic
12.4.5.1 Application
1. Mainly for charging traction batteries. If 10 hours charging time is
available, then charging is conducted according to the Wa characteristic; if
only 7–9 hours are available, then the WOWa characteristic must be

applied.
2. For charging small lead batteries with W characteristic with manual
deactivation.
3. For charging centralized batteries of safety-lighting equipment in alter-
native charging operation to WOW characteristic.
12.4.5.2 Basic Demands
1. For protection of the battery the W charact eristic must not allow the
current limit values determined by the manufacturer to be exceeded at
gassing voltage and at the end-of-charge voltage. The limit current values
upon reaching gassing voltage are listed in DIN 57 510/VDE 0510,
paragraph 12.2.3 (3), and in Table 12.3
2. The battery has to be disconnect ed manually (W) or automatically (Wa)
upon reaching the fully charged state.
Figure 12.3 Charging time for lead-acid and NiCd batteries. (A) Rectifier nominal current
for charging traction lead-acid cells GiS and PzS at 208C (688F) after discharge of (a) 80% and
(b) 100% of C
5
. (B) Rectifier nominal current for charging of stationary lead-acid cells OPzS,
Gro, GroE at 208C per 100 Ah K
5
after discharge of (a) 80% and (b) 100% of C
5
(operation
conforming to DIN 40729). (C) Rectifier nominal current for charging R-, TN/TS-, and
F-type cells at 208C per 100 Ah C
5
after discharge of (a) 80% and (b) 100% of C
5
(operation
conforming to DIN 40729).

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12.4.5.3 Characteristic
1. Progression (see Figures 12.1 and 12.2). Flow of the W characteristic is
determined by three pairs of values:
Nominal current of the device at 2.0 V/cell.
50% nominal current of the device at 2.4 V/cell.
25% nominal current of the device at 2.65 V/cell.
A tolerance of + 0.05 V/cell is permitted.
2. Nominal current of the device, charging current, and charging time
(Figure 12.3). Flow of the W characteristic allows a nominal current of
Figure 12.3 Continued.
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the device ( ¼ charging current at 2.0 V/cell), which is twice as high as the
allowed charging current at gassing voltage.
The initial ch arging current lies a bit lower than the device’s nominal
current as the battery voltage increases rapidly to 2.1 V/cell.
The nominal device current (I
L
) related to 100 Ah is to be respected for
the presently standardized lead-acid batteries (4). The device’s nominal
current I
dn
leads to the following equation:
Idn ðAÞ¼I
L
ðAÞ6
nominal capacityðAhÞ
100ðAhÞ
If shorter charging times are demanded than the W characteristic allows
for, then a higher device current can be adjusted, but must be limited to the

battery-specific limit value for the charging current (see Table 12.3) above
gassing voltage of 2.4 V/cell. This method corresponds to the WOWa
characteristic (see Figure 12.2). Best efficiency (ratio of costs for the
charger/charging time) is attained for a nominal device current of 32 Ah.
The shortest charging time is attained with a nominal device current of
40 A per 100 Ah.
3. Influence of the mains voltage. Charging currents from chargers with a W
characteristic are generally dependent on the mains voltage, which means
the device’s current yield is influenced by fluctuations of the mains voltage.
A mains voltage increased by 5% increases the current by 25% at 2.0 V/cell,
by 35% at 2.4 V/cell, and by 50% at 2.65 V/cell. Therefore the mains
voltage must be closely observed upon reaching the gassing voltage of
2.4 V/cell. In order to prevent damage to the battery, the charging device
must be adjusted to the augmented mains voltage (e.g . at night) by means
of a step-down transformer.
12.4.5.4 Guidelines for Operation
1. The resistance of the cables between the charger and the battery may
influence the gradient of the characteristic curve and therefore the charging
current. The length of these cables must therefore be considered (generally
the charging devices are adjusted to a certain cable length).
2. Charging of two or more batteries in parallel operation with a W-type
charger is not allowable since the current limit is not guaranteed for each
battery.
3. Charging in series operation is only allowable if the current value does not
exceed the smallest battery’s charge accepta nce capability and the fully
charged batteries are switched off in time.
4. Chargers with a W characteristic have only to be dimensioned thermally
for 80% of the nominal device current (because of the course of the
charging process). Therefore it is advisable to determine the nominal
current by the following equation (see also Section 12.4.5.3):

Idn ¼ I
L
6
nominal capacityðAhÞ
100ðAhÞ
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If a smaller nominal device current is chosen (for instance when there is
enough time), this has to be respected for dimensioning the device.
12.4.6 Charging Lead-Acid Batteries Corresponding to the
I Characteristic
12.4.6.1 Application
1. Mainly for charging starter batteries and batteries of the GroE type
according to the la characteristic.
2. Especially suitable for initial c harging (activation charging) following the
la characteristic.
3. For charging batteries of the GiS and PzS type according to the IOIa
characteristic.
12.4.6.2 Basic Demands
1. For protection of the battery, the I characteristic has to prevent the
charging currents from rising above the manufacturer’s specifications
above gassing voltage. The limits for the charging currents are listed in
Table 12.3.
2. The battery has to be manually (I) or automatically (la) disconnected from
the charger upon reaching fully charged state.
12.4.6.3 Characteristic
1. Progression (see Figures 12.1 and 12.2). Charging is conducted with
constant current throughout the charging period followed by the
deactivation of the charger.
2. Charging device nominal current, charging current, and charging time (see
Figure 12.3). Course of the I characteristic allows a charging device current

( ¼ charging current at 2.0 V/cell) of the same magnitude as the permitted
charging current at gassing voltage (see Table 12.3). For the batteries of the
GiS and the PzS type charging with a constant current (I, la) value results
in excessively long charging times because the current values are very low,
as Table 12.3 shows.
In order to attain acceptable values in this regard, the initial current is
increased until gassing voltage is reached, so an IOl characteristic is
formed. For batteries of the GroE type and starter batteries , acceptable
charging times are attained with the la characteristic.
3. Mains voltage influence. For simple controlled charging devices the
constant current changes proportionally with the fluctuations of the mains
voltage. This must be considered.
12.4.6.4 Guidelines for Operation
1. Parallel charging of batteries is not recommended as after gassing volta ge
has been exceeded, limitation of the current value must be guaranteed for
every battery (see Table 12.3).
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2. Series charging of starter batteries is common. Several batteries are series-
connected dependent on the charging devices’ voltage. Fully charged
batteries are disconnected and reconnected when discharged.
12.4.7 Charging of Lead-Acid Accumulators According to the
IUIa Characteristic
12.4.7.1 Application
1. Mainly for charging traction batteries when shortest possible charging time
(below 8 h) is demanded.
2. For ‘‘heedful’’ charging.
12.4.7.2 Basic Demand
1. For protection of the battery the charging current has to be reduced in the
second I period so it does not exceed the manufacturer’s limits above
gassing voltage.

The limit values for the charging cu rrents above gassing voltage are
listed in Table 12.3. During the constant-voltage charging period (2.4 V/
cell) an allowed tolerance of + 1% may not be exceeded (5).
2. The battery must be disconnected manually (IUI) or automatically (IUIa)
after the fully charged state has been attained.
12.4.7.3 Characteristic
1. Progression. Charging is initially co nducted with constant current until
gassing voltage is reached (first I part). Then the voltage is kept constant
(U part), and the current decreases permanently as the battery’s state of
charge increases. As soon as the current has dropped to the allowed end-
of-charge current (see Table 12.3), the final charging phase (second I part)
is activated. Upon reaching the fully charged state, the battery has to be
disconnected.
2. Nominal device current, charging current, and charging time. The charging
current for the first I period does not have to be limited by the charging
device. This is done, however, to protect the charging device and the
equipment (charging cables, etc.). Charging with the IUIa characteristic
permits charging times below 8 hours. Char ging currents of twice or three
times I
5
are not of interest as the gassing voltage is attained too fast and
forces lowering of the charging current, and apart from that makes the
chargers more expensive. The most economic solution is a nominal device
current of 25 A per 100 Ah. The shortest charging time is attained with
40 A charging current per 100 Ah.
3. Mains voltage influences. The given tolerances of the characteristic curves
(constant voltage +1%, constant current +2%) have to be guaranteed for
mains fluctuations of +10% and frequency fluctuations of +2%. For
devices that are not dimensioned to cope with these fluctuations, the
manufacturer must specify the allowed ranges. If greater mains fluctua-

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tions of private networks are to be expected, this must be mentioned when
ordering the charging device.
12.4.7.4 Guidelines for Operation
Parallel charging of batteries is not recommended in the second I period for the
charging-current limit must be guaranteed for all of the batteries charged (see
Table 12.3). Devices with a possibility to switch over from IU to IUIa can be
employed for parallel charging in the IU position.
12.4.8 Charging According to the IU characteristic
12.4.8.1 Application
1. Mainly for charging traction batteries, whereas IU-type charging is
conducted in large battery stations for parallel charging of several batteries
at a time.
2. For fast charging of batteries (in pauses, for increase of the operational
time), whereas fully charged state is not attained.
3. For heedful charging of sulfated batteries. (The voltage does not exceed
2.4 V/cell; the initial current is low and only increases when the sulfatation
has been reduced. Hereby the voltage even decreases for a transitory period
of time. With an uncontrolled charging device the voltage would
immediately rise above 2.4 V/cell , the initial current would be higher, the
battery would start to gas strongly, and the temperature could rise to
unallowed values.)
12.4.8.2 Basic Demand
For protection of the battery, the voltage of 2.4 V/cell must be kept constant
within +1% during the U phase (5).
12.4.8.3 Characteristic
1. Progression. Initial charging is conducted with constant current (I section)
until gassing voltage is reached. Upon reaching the gassing voltage the
charging device’s voltage is kept constant (U section) and the charging
current decreases. Fully charged state is only attained after a longer period

of charging.
2. Device’s nominal current, charging current, and charging time. The charging
current during the I period would not have to be limited because of the
battery, but only for protection of the charger and the equipment (charging
cables, etc.). The battery is also indifferent to the tolerance of current
limitation, but the smaller the tolerance, the better, as the charger’s electric
power is optimally employed and thus the ch arging time shortened
(especially for simultaneous charging of several batteries). State of charge
of 100%, respectively 80%, is attained by partial charge in very short
periods of time. (Fully charged state corresponds to 120%, respectively
100%, with a charging coefficient of 1.2.)
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3. Mains voltage influences. The given tolerances of the characteristic curves
(constant voltage +1%, constant current +2%) have to be guaranteed for
mains fluctuations of +10% and frequency fluctuations of +2%. For
devices that are not dimensioned to cope with these fluctuations, the
manufacturer must specify the allowed ranges. If greater mains fluctua-
tions of private networks are to be expected, this must be mentioned when
ordering the charging device.
12.4.8.4 Guidelines for Operation
1. Batteries may remain connected to chargers of the IU type for up to 3 days
if the final value of the charging current does not exceed 10% of I
5
. If the
final charging-current value is higher, then charging must be interrupted
upon reaching the maximum permissible electrolyte temperature of 358C
(1318F).
2. If the available daily charging time is not sufficient during the week, an
equalizing charge has to be conducted once a week. If the batteries are fully
charged during the day, then one equalizing charge every 4 weeks is

sufficient. This equalizing charge can be conducted by a prolonged
charging according to the IU characteristic during a weekend (guideline 1
must be respected!) or through charging with an increased voltage above
gassing voltage (2.4 V/cell) if the charging devices are equipped with the
corresponding device (such as an IU-IUW-IUI switch). The necessity for
single charging during the W phase (for IUW) or the second I phase (for
IUI) must be respected when parallel charging is conducted and the limit
charging-current values listed in Table 12.3 are not exceeded.
3. When dimensioning the cables connecting the battery to the charger, a
voltage drop of less than 2% should be realized for nominal current.
4. When charging according to the IU characteristic, the electrolyte gravity
and temperature of every cell have to be checked once a week in order to
notice shorts between plates in time.
5. If a battery of low capacity is charged with a charging device of strong
nominal current, then the electrolytes’ temperature has to be surveyed
above 2.4 V/cell or the charging current has to be reduced.
12.4.9 Charging of Nickel/Cadmium Batterie s
NiCd batteries are generally charged according to one of the following three
charging methods: I (la)-, W (Wa)-, or IU-type charging.
Charging according to the IOIa, WOWa, and IUIa characteristics is of course
possible but not common as these characteristics are not necessary for charging
NiCd accumulators (except IOIa-type charging for cells with sintered electrodes).
12.4.9.1 Basic Demands
1. NiCd batteries (except sintered cells) do not demand limitation of the
charging current. For protection of the battery, however, it must be
guaranteed that the limits for the electrolyte temperature (458C (1138F);
for pocket-plate cells 358C (958F)) are not exceeded.
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2. Batteries have to be disconnected upon reaching fully charged state.
3. Equalizing charging with a current value of 15 should be conducted for 15

hours every 2 to 3 months for NiCd batteries.
12.4.10 Charging of Nickel/Cadmium Batteries to the
I Characteristics
12.4.10.1 Application
Suitable for all designs.
12.4.10.2 Characteristic
1. Progression. The charging-current value is kept constant throughout the
charge and the battery must be disconnected manually (I) or automatically
(la) upon reaching fully charged state.
2. Device’s nominal current, charging current, and charging time. Currents of
0.5 to 1.5 times I
5
are applied. For NiCd cells wi th sintered electrodes, the
charging current must be limited ab ove gassing voltage (see Table 12.1).
This causes long charging periods for cells with sintered electrodes. In
order to attain shorter charging times, the initial charging current (up to
the gassing voltage) is augmented, forming an IOI (IOIa) characteristic.
For NiCd batteries of the R, T, and TS types, acceptable charging
times are attained when applying a current of 1.5 6 I
5
.
3. Mains voltage influences. For regulated charging devices of simple design,
the constant current changes proportionally to the mains fluctuations.
12.4.10.3 Guidelines for Operation
The charging current must be reduced for high electrolyte or ambient temperatures
(greater than 458C (1138F)).
12.4.11 Charging Nickel/Cadmium Batteries According to the
W Characteristic
12.4.11.1 Application
For all types of constructions and designs this is the most common and heedful

charging method (temperature and water consumption are kept especially low).
12.4.11.2 Characteristic
1. Progression for series R, T, and TS NiCd cells. As these series exhibit
different initial voltage (1.25 to 1.5 V/cell), a range for the characteristic
was fixed for which the characteristic of the charger must be adjustable (6).
The adjustment in this range can be accomplished either by adjustment of
the characteristics’ niveau or gradient. As the gradient adjustment is of
greater technical expenditure, the niveau adjustment method is more
commonly applied. This kind of adjustability, however, is not demanded
for the charging device when the charger is only applied for one type of
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battery, which is generally practiced. Development has lead to the initial
voltage generally being 1.4 V/cell as the voltage of lower-situated battery
voltage levels rises quickly to this value when charged.
Two pairs of values therefore determine progression of the
characteristic: (a) device’s nominal current at 1.4 V/ cell and (b) 40 to
50% of the devices’ nominal current at 1.75 V/cell.
2. Progression for series F NiCd cells. For charging NiCd batteries with
sintered electrodes a characteristic according to DIN 41 774 (W-type
charging for lead-acid batteries) is employed with the following values:
Device’s nominal current (DNC) at 1.2 V/cell.
About 40% of DNC at 1.5 V/cell.
About 25% of DNC at 1.6 V/cell.
3. Progression for Ni/Fe batteries. Series RE and TN E also have different
initial voltages (1.5 and 1.75 V/cell); therefore a range was determined in
which the characteristic must be ad justable (6). Adjustability is also not
demanded when the charger is only applied for one type of battery. The
progression for an adjustable characteristic is determined by the following
values: (a) DNC for 1.5 to 1m75 V/cell and (b) 40 to 50% of DNC for 1.65
to 1.9 V/cell.

Ni/Fe batteries only attain fully charged state if the charging current
does not drop below one-third of I
5
in the course of charge.
4. Device nominal current, charging current, and charging time. Adjustment of
the DNC ( ¼ initial charging current) for type and size of the battery is
determined by the time available for recharge. Standard value for the
charging current is 0.5 to I
5
times I
5
. The electrolytes’ temperature is kept
within acceptable limits for these charging currents.
5. Mains voltage influences. For charging devices with W characteristic the
current value is generally influenced by mains fluctuation. Variance of 10%
of the mains voltage results in 30 to 50% variance of the charging current.
The battery is indifferent to these changes of c urrent, but the charging
device is not. Therefore in the case of mains fluctuations over longer
periods of time, step-down transformers must stabilize the charger.
12.4.11.3 Guidelines for Operation
Parallel charging of batteries cannot be advised for W-type chargers as varying
battery voltage levels result in unequal charging of the batteries an d therefore in an
uneven state of charge. For Ni/Fe batteries the lower current limit is not guaranteed
during recharge.
12.4.12 Charging of NiCd Batteries According to the IU
Characteristic
12.4.12.1 Application
This charging method is only employed in a very few cases for single-battery
charging, but is mostly employed for parallel recharge of several batteries.
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12.4.12.2 Characteristic
1. Progression. Charging is conducted with constant current (I section up to
gassing voltage). After the gassing voltage has been reached, the device’s
voltage is kept constant (U part) and the current values decrease to lower
values.
Upon reaching the fully charged state the battery has to be
disconnected. This constant voltage having a tolerance of +2% (DIN 41
772) lies between 1.6 and 1.7 V/cell depending on the type of cell (2).
2. Device’s nominal current, charging current, and charging time. The charging
current (DNC for the first I section) is only limited by the electrolyte
temperature. Charging currents of 1.5 to 2 times I
5
show good results for
the charging time.
3. Mains voltage influences. The given tolerances of the characteristic curves
(constant voltage +2%, constant current +2%) have to be guaranteed for
mains fluctuations of + 10% and frequency fluctuations of +2%.
12.4.12.3 Guidelines for Operation
1. When parallel ch arging is performed, only batteries with the same number
of cells and of the same type are connected.
2. For high ambient and electrolyte temperatures (greater than 458C (1138F))
the charging current must be reduced.
12.4.13 Charging Valve-Regulated Lead-Acid Batteries
12.4.13.1 Charging Methods
Two methods are commonly practiced: (a) charging according to the W
characteristic and (b) charging according to the IU characteristic. The W method
is not advisable as the charge current is dependent on mains fluctuations.
12.4.13.2 Charge Currents and Charging Time
Charging data are given in Table 12.4
Table 12.4 Comparison of charging data for W- and IU-type charging characteristics.

Characteristic
Charging
current
Charging
voltage
a
Charging
time
Filling
ratio Operation mode
W Max. 2 6 I
20
2.3 V/cell ca. 14 h 90% Cyclic operation
IU 2–10 6 I
20
2.25–2.3 V/cell 14–4 h 90% Float-charge operation
(parallel operation)
a
ambient temperature 208C (688F)
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12.4.14 Charging Gas-Tight Nickel/Cadmium Batteries
12.4.14.1 Charging Methods
The two following methods are generally applied:
1. Charging according to I (Ia, IOIa) characteristic.
IOIa characteristic;
3 to 10 times I
10
(series dependent) to end-of-charge voltage.
Half of I
10

to I
10
to end of charge (full charging).
2. Charging according to W (Wa, WOWa) characteristic.
W characteristic;
1.2 6 I
10
initial charge current.
0.8 I
10
end-of-charge current.
Charging according to I and W characteristics without deactivation is only allowed
for currents 0.1 to 0.3 times I
10
(series dependent).
When charging with higher current values, deactivation of the charge process
must be controlled as it is time-, voltage-, and tempe rature dependent.
Charging according to an IU characteristic is prohibited.
12.4.14.2 Application
The option for applying charging methods according to the I or W characteristics,
respectively their modifications, can be derived from Figure 12.2.
12.4.14.3 Charging Currents and Charging Time
Charging data are given Table 12.5.
12.5 COMPARING CHARGING METHODS FOR LEAD BATTERIES
Table 12.6 compares some charging methods for lead batteries
Table 12.5 Charging data for gas-tight NiCd batteries.
Type of charge Charging current Charging period Characteristic Filling ratio
Standard charge I
10
14 hrs Ia 100%

Accelerated standard 2 À 3 6 I
10
7 À 4.5 hrs Ia 100%
charge (series-dependent)
Fast charge 3 À 10 6 I
10
2.5 À 1 hrs Ia 70–90%
(series-dependent)
Float charge 0.1 À 0.3 I
10
unlimited I
(series dependent)
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12.6 INSTALLATION COSTS OF CHARG ING DEVICES
The costs for IU-type charging devices contain costs for one secondary charger for
every three parallel charged batteries and for two secondary chargers for every three
to eight batteries. The following comparison is on batteries 250 Ah, 80 V ( ¼ 40 cells).
The battery-dependent charging currents of the charging devices are listed in
Table 12.7.
Figure 12.4 shows a comparison between the initial costs for chargers of
different characteristics.
12.7 GUIDELINES FOR THE SELECTION OF CHARGERS
This comparison of the charging method (Figure 12.3) and the initial costs (Section
12.6) allow for judgment of which devices are to be applied.
Device characteristics are to be chosen, when:
12.8 SPECIAL DEMANDS AND RECOMMENDATIONS FOR THE
CHOICE OF CHARGER
12.8.1 Demands of Valve-Regulated Lead-Acid Batteries
To avoid gassing charge current, charge voltage and charging time are limited.
12.8.1.1 Charging Characteristic of Traction Batteries with VRLA Cells

The charge characteristic is specified by the battery manufacturer. Mostly this
characteristic is a modification of the above-described IUI characteristics. The initial
charge current is between 0.7 to 1 times I
5
, the final charge current between 0.07 to
0.08 times I
5
(a following additional charge sometimes is performed in current
pulses). The total charging time is between 11 and 14 hours. Standardized
characteristics do not yet exist.
Charger manufacturers offer chargers allowing the charge of batteries designed
by different manufacturers (the chargers are controlled by a microprocessor that
recognizes the characteristics specified by the battery manufacturer).
Wa: (a) The mains voltage fluctuations are less than +5%.
(b) A charging time of 10–12 hour s is available.
WOWa: (a) The charging time is limited to 7–9 hours.
(b) The mains voltage fluctuations are less than +5%
IUIa: (a) Char ging time is limited to 6–7 hours.
(b) The mains voltage fluctuations are significant.
(c) Heedful charging is desired.
(d) The high initial costs are acceptable.
IU: (a) Parallel charging of batteries is to be conducted.
(b) Short partial charging (e.g. at noon) is desired.
(c) The mains voltage fluctuations are significant.
(d) Additional secondary, respectively equalizing, charging is acceptable.
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12.8.2 Demands of Modified Traction Batteries
Note: Traction batteries of the modified type are equipped with a device for electrolyte
circulation).
To increase the service life and to reduce maintenance to a minimum a heedful

charge of the batteries is recommended. This can be performed by electrolyte
circulation (reduced charging factor to 1.04 to 1.08, and reduced battery temperature
means a double effect) or cooling with water or a cooling device (to reduce the
battery temperature).
12.8.2.1 Charging of Modified Traction Batteries
The above-described charge characteristics Wa, WoWa, IU, and IUIa may be
performed (depending on the battery manufacturer’s advice for the modified form).
The initial charge current is chosen depending on the charging time wanted.
The complete charging process needs in addition to the battery;
1. Electrolyte circulation. Charger with the specified characteristic and a
correct charging factor (1.04 to 1.08), and a pumping device for the
electrolyte circulation with a control set.
2. Cooling with water. Charger with the specified characteristic and a cooling
device with connection to a fresh water supply.
3. Cooling device. Charger with the specified characteristic and cooling device.
Depending on the battery manufacturer these variants are offered as a complete
system.
Figure 12.4 Initial costs for charging devices with different characteristics compared to the
initial costs for a Wa-type charger (cost for Wa-type charger ¼ 1).
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Table 12.6 Comparison of charging methods for lead batteries.
Wa WOWa IUIa IU
Device nominal current Device nominal current Device nominal current Device nominal current
Progress of
charge
0.8 6 I
5
, current decreases
with rising voltage
1.6 6 I

5
, after gassing voltage
of 2.4 V/cell is reached
automatic changeover (0) to
Wa characteristic
(1.5 to 2 6 I
5
) constant up to
gassing voltage of 2.4 V/cell.
Then automatic changeover
to constant voltage until
current has decreased to
0.2 6 I
5
, then automatic
changeover to the Ia phase.
Charging conducted with
constant current until
deactivated.
(1.5 to 2 6 I
5
) constant up to
gassing voltage of 2.4 V/cell,
then automatic changeover
to constant current
Charging 10–12 h 7–9 h 6–7 h 2.5–3.5 h for 80% full charge
with secondary charger 10–
12 h
Advantages Upon reaching fully
charged state automatic

deactivation (a); simple
technology
Short charging periods; upon
reaching fully charged state
automatic deactivation (a);
simple technology
Shortest charging period;
especially heedful charging
of the battery; mains voltage
fluctuations (+10%) are
without effect on the
charging current
Partial charges up to 80% in
very short periods of time;
parallel charging of several
batteries possible; less water
loss through gassing; mains
voltage fluctuations (+10%)
are without effect on the
charging current; no time-
dependent deactivation
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Wa WOWa IUIa IU
Disadvantages Only single charging
possible; charging current
generally varies
substantially with mains
fluctuations
Only single charging possible;
charging current varies

substantially with mains
fluctuations
Only single charging possible,
but parallel charging also
possible with additional
devices
Very low gassing action during
daily charge, therefore
unfavorable mixture of the
electrolyte. Equalizing
charging necessary during
the weekend; in some cases
secondary charger is
necessary; If one IU charger
is employed for
simultaneously charging
several batteries a
breakdown has more severe
consequences than if several
chargers were employed.
Table 12.6 Continued.
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REFERENCES
1. DIN Standard, DIN 40 729. Accumulators, Definitions.
2. DIN Standard, DIN 41 772. Rectifiers, Semiconductor Devices.
3. DIN Standard, DIN 57 510/VDE 0510.
4. DIN Standard, DIN 41 774. Chargers with W Characteristics for Lead-Acid Batteries.
5. DIN Standard, DIN 41 773. Part 1, Chargers, Rectifiers with IU Characteristic for Lead-
Acid Batteries.
6. DIN Standard, DIN 41 775. Chargers with W Characteristic for NiCd and NiFe Batteries.

7. Bechthold, Leander, Ladezeiten. Publication by Industrie-Automation.
Table 12.7 Comparison of charging devices with different charging
characteristics for a charging current for 40 cells with 250 Ah.
Wa characteristic 0.8 I
5
¼ 40 A
WOWa characteristic 1.66 I
5
¼ 80 A
IU characteristic 1.8 6 I
5
¼ 90 A
IUIa characteristic 1.8 6 I
5
¼ 90 A
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